Sign Reversal of the Quantum Hall Number in (TMTSF)2PF6

The quantum Hall effect is investigated in the organic conductor (TMTSF${)}_2$P${\mathrm{F}}_6$ (TMTSF is tetramethyltetraselenafulvalene) under hydrostatic pressure. Under a pressure of $p=9\mathrm{}$ kbar, the sequence of the quantum Hall numbers $L={\sigma }_{\mathrm{xy}}/\left({-2e}^2/h\right)$ with increasing magnetic field is regular: $L=\dots ,3,2,1,\dots$ as reported earlier. At $p=8.5\mathrm{}$ kbar, however, the irregular sequence $L=\dots ,3,-2,2,1,\dots$ is found. The temperature–magnetic field phase diagram is investigated in detail. We discuss our findings in the framework of recent theories of the quantum Hall effect in magnetic-field-induced spin-density waves.